//要能判断自己是否已经被实例化过
//实现单例模式
class SingleDog{
    show (){
     console.log('我是一个单身狗');
    }
    static getInstance(){
      if(!SingleDog.instance){
        SingleDog.instance =new SingleDog()
      }
      return SingleDog.instance
    }
}
const s1 =SingleDog.getInstance()
const s2 =SingleDog.getInstance()
console.log(s1 === s2);
